One of our most popular posts was about the Stinker Station signs that once dotted the landscape in southern Idaho. The one everyone seems to remember said, “Petrified watermelons. Take one home to your mother-in-law.” Those melons were actually rocks, and if a few people followed the sign’s advice, no one cared. There were plenty of them.

So, what is melon gravel? It’s a common type of rock found from about Massacre Rocks State Park to the Oregon border. It is known for its shape more than its composition. Most melon gravel is basaltic rock that was torn away from the Snake River Canyon walls and lava flows during the Bonneville Flood some 15,000 years ago. The flood carried that rock along for miles, tumbling it against other rocks, knocking off the edges, until it began to round and become relatively smooth.The gravel ranges in size from, well, a melon, to an SUV. In places, you can find deposits of it a mile wide and a mile and a half long. Many fields in the Magic Valley have piles of the rock that has been scooped up over the years to make way for crops.

For more about the Bonneville Flood, check Terry Maley’s book, Exploring Idaho Geology, a new edition of which has just been released.